In a basic material industry related to each technical field including foods, cosmetics, ink, paint, adhesive, coating agent, fine chemical, medicine, new nanomaterials, and advanced electronic materials, generally, any one selected material and another material are mixed to be used as a basic material.
For the materials to be mixed as the base material, uniformity and fineness of the mixing by the processes of dispersion and emulsion of respective particles influence the quality of a finished product, and various types of dispersion and emulsion apparatuses which are suitable for these requirements have been developed and used.
Dispersion (homogenization) is a method in which the sizes of the particles are decreased, as a solid which contains powder is homogeneously mixed in a liquid or the liquid is homogeneously mixed in another liquid, and as a result, the particles uniformly exist in a stable state. And it may be divided into suspension in which “the solid” is mixed with “the liquid”, and emulsion in which “a first liquid” and “a second liquid” having an interface are mixed.
Therefore, the dispersion (homogenization) means to make the particle size smaller, and making the particle size smaller is to make the particle size smaller by applying strong energy (driving force) to a corresponding material for it to be ground, sheared, or cut.
A traditional dispersion and emulsion apparatus is a high-pressure type homogenizer, which makes the target materials collide with a wall or inverted by converting pressure (energy) to a jet stream to convert kinetic energy to shear energy, thereby achieving the dispersion and the emulsion. In such a scheme, there is a problem that pressure and velocity gradient exist in a reactant (materials to be mixed) in a processing procedure and air dissolved in contents thus generates bubbles, the mixed particles are uneven by the bubbles, and a long time is required for smooth dispersion and emulsion of the materials, and as a result, efficiency deteriorates.
Meanwhile, mixing will be discussed in more detail compared with dispersion.
The dispersion represents a state in which the particles of the materials are made very small and the particles are distributed uniformly and stably with each other. Unlike dispersion, mixing represents just mixing the materials (substances).
When a propeller and an impeller are rotated by using mixing equipment such as overhead mixer, two materials are mixed by the rotation of a blade of the impeller, and this is simply mixing where the homogenization process, which is the process of making the particle size small, is omitted.
A unit indicating a degree of particle size reduction in the mixing process is “shearing force”. And the difference between the mixing and the dispersing is very great as shown below.
Stirrer or MixerDisperserTip Speed0~10 m/s10~24 m/sClearance~250 mm5 mm or lessShearing Force0~402,000~4,8000Shearing Force = Tip Speed/ClearanceTip Speed: Instantaneous speeds of rotor and impellerClearance: The gap between rotor and stator or the gap between impeller and vessel wall
Meanwhile, a largest issue in performance of the dispersion is viscosity of a material to be dispersed. Dispersion efficiency deteriorates at the viscosity of approximately 2000 mPas or above, and when the viscosity becomes 5000 mPas, dispersion using a general apparatus is not performed.
For materials with viscosity, the shearing force should be increased by increasing the rotational speed of the impeller to increase the tip speed of the rotor, and by installing fence to reduce the numerical value of the clearance or the gap.
FIG. 1 is a functional configuration diagram of an ultrasonic dispersion-emulsion apparatus by an embodiment of conventional technology. The ultrasonic dispersion-emulsion apparatus may be described with reference to the accompanying drawing. The ultrasonic dispersion-emulsion is a scheme that when 20 kHz ultrasound is emitted into a solution with a strong intensity, numerous microcavities are generated in the solution, and when the microcavities are broken, shock wave energy with a high temperature and high pressure is generated, and the particles of the material to be dispersed are broken to be very small (fine) by the shock wave energy.
Since the dispersion-emulsion scheme using the ultrasound is very effective, there is advantage that it can enable nanoscale dispersion-emulsion, but a long working time is required for a material with viscosity and it has a limitation in the uniformity and homogeneity of the mixture.
FIG. 2 is a functional configuration diagram of a rotor-stator type dispersion-emulsion apparatus by an embodiment of conventional technology.
To explain the dispersion-emulsion apparatus with reference to the accompanying drawing, it is a scheme in which the dispersion-emulsion apparatus is constituted by a fixed stator and a rotating rotor, and the particles of the target materials (substances) passing between the rotor and the stator are finely broken by strong shearing force generated by strong rotational energy to achieve the dispersion-emulsion.
The tip speed of the rotor which rotates at a speed of tens of thousands of rpms by a motor having strong rotary force reaches approximately 20 m/sec, and the material to be dispersed passes between the rotor and the stator at a tremendous speed. In this case, the clearance between the rotor and the stator, that is, the gap between the rotor and the stator forms a very small gap of approximately 0.1 mm, and when the material to be dispersed passes through the small gap between the rotor and the stator at a strong rotational speed, tremendous sharing force is generated and the particle is thus instantaneously sheared or cut into a very small size.
Such a rotor-stator type was developed decades ago and IKA of Germany had owned a patent right on such a rotor-stator type for a long time, and at present, the method has already been opened as the patent expired.
An advantage of such a scheme is that high-viscosity dispersion is possible, a dispersion-emulsion effect is excellent in terms of convenience of a process. Currently, the rotor-stator type dominates the main stream of most dispersion-emulsion apparatus, commercialized equipments are already in the market, though there are differences of performances from product to product.
However, for the dispersion-emulsion of micro-particles of nanoscale, there are still some problems. For example, mixing is not well performed and long processing time is required.
Therefore, it is necessary to develop more advanced technology so as to increase the shearing force to homogenize the particles and improve the uniformity of the particle size, and to improve the processing time for the dispersion-emulsion of the nanoscale microparticles.